Wind turbine blades for use in large scale wind turbines on wind farms are becoming ever bigger. For example, blade lengths may be over 50 m. New designs of these ever larger blades are typically mechanically fatigue tested before being put into use to ensure that they will be reliable for the service life of the wind turbine.
In normal use, wind turbine blades oscillate as they rotate as the forces acting on them change. Wind turbine blades are subjected to two types of load namely aerodynamic load (in the form of, for example, lift, drag and shear) and inertial load (in the form of, for example, gravity and blade dynamics). Loads are seen in both the flapwise and edgewise directions of a wind turbine blade 10 as illustrated in, for example, FIG. 1. The edgewise direction is the direction of rotation of the wind turbine blade, in use, and the flapwise direction is perpendicular to this and also perpendicular to the longitudinal axis of the blade. Aerodynamic bending moments are generally associated with the flapwise direction. These are mainly attributed to stochastic wind speed, that is to say, turbulence. The inertial loads are generally associated with the edgewise direction. The loads are attributable, in the main, to gravity loads experienced during each rotation of the blade and are more deterministic in nature. In small blades, loads in the flapwise direction dominate the edgewise loads. Thus, small blades can be adequately tested only in one direction (flapwise). However, with today's larger blades, the edgewise loads become more significant as a blade's gravity load increases greater proportionally than its wind load with blade length. Therefore, for larger blades, single axis testing does not provide a valid test and biaxial testing is required.
Blade testing is typically done by exciting the blade at its natural frequency (the frequency a system naturally vibrates once it has been put into motion). In this way, it is possible to emulate the required test bending moments across the whole blade. Longer turbine blades have lower natural frequencies and test times increase as blades are usually required to be tested for a particular number of oscillations. Furthermore, the edgewise and flapwise natural frequencies are different. The edgewise natural frequency of a wind turbine blade is higher than the flapwise natural frequency. A typical test time for which a blade might be oscillated continuously at the natural frequency in each direction is of the order of 3 months. Adding to this burden, newer, longer blades are increasingly flexible, which requires more energy to oscillate them.
In a simple test arrangement (not shown), typically housed in a large building to cover the blade being tested, one end of a wind turbine blade (that which would, in use be connected to the hub of the wind turbine) is fixed to a substantial concrete support that may weigh several thousand tons to support the substantial loads applied to the blade during testing. The other end of the blade is oscillated in one direction to test it in that direction by applying an oscillating mass to the upper surface of the wind turbine oscillating at the natural frequency of the blade in the direction it is oscillating, for example, for three months. The oscillating mass is typically a mass mounted to a rotating electric motor. Once the test is completed, the wind turbine blade is rotated 90° and an oscillating mass is applied to what is now the upper surface of the wind turbine blade and the test is repeated at a different frequency corresponding to the natural frequency of the blade in the direction it is now oscillating again for, for example, three months.
A number of biaxial fatigue testing arrangements are known to test the blade simultaneously in two directions (flapwise and edgewise). Clearly, this would reduce the overall test time of the wind turbine blade. However, correct control is extremely difficult. This is because, as mentioned above, the blade should be oscillated at different frequencies in the edgewise and flapwise directions. Typically, the oscillation in one of these directions affects the oscillation in the other direction.
US patent application No. US2006/0037402 describes one arrangement that allows for simultaneous biaxial testing. The mechanism for applying flapwise loads includes an actuator and a mass mounted on the wind turbine blade being tested. A control system controls the actuator to linearly reciprocate the mass in the flapwise direction at about the resonant frequency of the wind turbine blade being tested. Another mechanism applies edgewise loads to the wind turbine blade being tested. This mechanism includes another actuator coupled to the wind turbine blade being tested by a pivotally-mounted bell crank and a pushrod. The control system controls edgewise testing too.
The use of a heavy oscillating or reciprocating mass on the blade increases the mass of the blade being tested, which increases the natural frequency of the blade being tested. Furthermore, this arrangement is inefficient and difficult to control because the angle of the pushrod is always changing over time. This testing system is also large in size.
Another arrangement for simultaneous biaxial testing is described in international patent application No. WO 2009/097055. In this arrangement, the wind turbine blade under test is mounted to a frame. Actuators moving perpendicular to one another act on the frame on different sides of the blade. One actuator provides flapwise loads by applying force to a lever arm or fin connected to the wind turbine blade being tested and causing the wind turbine blade to oscillate. The other actuator provides edgewise loads by applying a force to cause linear displacement. The actuators are operated by a control system with displacement provided at natural or other frequencies. The control system uses a feedback loop that receives data from sensors, such as strain gauges, located on the flap and edge portions of the blade.
This arrangement requires large forces to be provided by the actuators in the edgewise direction, in particular, and like the other prior art arrangement described above, it is difficult to control and is large in size.
Both of the biaxial test arrangements of US patent application No. US2006/0037402 and international patent application No. WO 2009/097055 described above have actuators that act on the wind turbine blade orthogonal to one another.
The inventors of the wind turbine blade tester described below are the first to appreciate that a wind turbine blade and, in particular, a large wind turbine blade that might be used on a wind turbine on a wind farm, can be adequately tested in the flapwise and edgewise directions simultaneously using a pair of linearly reciprocable actuators, such as hydraulic actuators, each arranged to deliver a stroke to a wind turbine blade being tested and the stroke delivered by each of the actuators provides a controlled force in the edgewise and flapwise directions.
Wind turbine blades have low stiffness in the flapwise direction and, therefore, require high displacement in this direction to achieve the target bending moments and aerodynamic damping in this direction is the predominant factor. In contrast, the edgewise direction only requires forces to overcome the relatively low structural damping. Thus, required forces to overcome the above-mentioned forces are much greater in the flapwise direction than in the edgewise direction. This is provided by examples of the arrangement described herein. Running the test at the natural frequency of the wind turbine blade being tested means that resonance occurs (resonance is the build-up of large amplitude that occurs when a wind turbine blade is excited at its natural frequency), and the only force required is that to overcome the structural and aerodynamic damping of the blade. In this case, typically, the ratio of flapwise forces to edgewise forces is 10:1.
Examples of the wind turbine blade tester described herein provide, quick, efficient and reliable testing of wind turbine blades in two directions (edgewise and flapwise) simultaneously in a compact design. Examples require only low ground clearance below the blade being tested. Generally, this allows for better optimisation of wind turbine blade design and, as a result, shorter times to bring a new product to market. Example blade testers described can compensate for changes in actuator angle.
By exciting the blade being tested at its natural frequency, the required local bending moments can be achieved by mass distribution. This allows for low energy required per test and relatively low required excitation forces. As a result of the latter effect, the required relative strength of the test equipment is low, which reduces costs.
Examples of the wind turbine blade tester described herein also provide advantages as regards single axis testing. They restrain movement in the axis along which the blade is not being tested and this reduces cross-coupling between the edgewise and flapwise directions. The blade does not require pitching between edgewise and flapwise testing. Also, example turbine blade testers used in single axis testing also provide correct mean loads for edgewise testing.